Shipping rates different from Buyer to Seller?

I recently sold two items to separate buyers in the US. Each of them selected Fedex International Economy and paid about $23 CAD. When I go to ship the item that same shipping option, Fedex International Economy, is listed at $64. How can it be that a buyer gets a rate 1/3 of the rate I have to pay for the same shipping mode?

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What State were they going to?